This invention relates to a riser card topology for computer boards and, more particularly to a 66 MHz PCI bus riser card system capable of coupling one or more peripheral boards to a PCI bus.
As computer systems evolve, there is an increasing need to provide more components, such as semiconductor devices, connectors, memory, etc., per square inch of board space. In some applications, such as web servers, part of this need has been addressed by employing a plurality of chassis arranged within a rack. A chassis is a device designed to house or hold a computer board or circuit board. A plurality of chassis may then be closely arranged or stacked in a rack, which is often a cabinet-like structure. The rack provides a common bus on to which the computer boards may be connected for operation.
In order to provide certain design standards, chassis enclosures are specified as 1U, 2U, etc. xe2x80x9c1Uxe2x80x9d defines a chassis that can house a circuit board no more than 1.75 inches high. xe2x80x9c1Uxe2x80x9d systems are often deployed in data centers and host service provider sites to achieve maximum performance density. xe2x80x9c2Uxe2x80x9d is defined as xe2x80x9c2xc3x971Uxe2x80x9d or 3.5 inches, and so on. As a consequence, these units of separation define the maximum height of computer boards, including all components and peripheral cards that can be deployed in such chassis.
One way in which such performance density is achieved is by minimizing the number of connectors or sockets that are mounted on a board. However, this also limits the number of cards that may be connected onto the board.
A significant problem also exists as a result of the limited spacing between each computer board. For instance, in a 1U system, a mere 1.75 inches in height are available to fit memory modules and peripheral cards. Because many connectors and sockets on a computer board are mounted vertical to the surface of the board, this prevents installing many peripheral cards as they would protrude beyond 1.75 inches in height.
One solution to this problem is to use a riser card which permits installation of peripheral cards in a horizontal position to the computer board. Generally, a xe2x80x9criser cardxe2x80x9d is a module which couples to an existing connector or socket on the computer board and includes one or more connectors on the module onto which peripheral devices may be coupled. By making a riser card compact and mounting the connectors thereon in a horizontal position to the computer board, peripheral cards may be coupled to the riser card so as to fit within the 1.75-inch constraint of 1U systems.
One common connector for peripheral cards is a Peripheral Component Interconnect (PCI) socket. The PCI bus was proposed by Intel Corporation as a solution to provide a high-speed expansion bus standard. The original PCI bus standard has been upgraded several times, with the current standard being Revision 2.2, available from a trade association group referred to as PCI Special Interest Group, 5440 Westgate Drive, Suite 217, Portland, Oreg. 97221; also found at www.pcisig.com. The PCI Specification, Rev. 2.2, is incorporated herein by reference. The PCI bus provides for 32-bit or 64-bit transfers at 33 MegaHertz (MHz) or 66 MHz. It can be populated with peripheral cards, including adapters, requiring fast access to each other and/or with system memory, and can be accessed by the host processor at speeds approaching that of the processor""s native bus speed. A 64-bit, 66 MHz PCI bus has a theoretical maximum transfer rate of 528 MegaBytes/sec.
As the demand for faster computers has grown, the 66 MHz PCI bus has become common in computer systems. One consideration in the design of computer boards employing the 66 MHz PCI bus standard is signal reflection. Signal reflection occurs where a transmission signal crosses from a first transmission medium to a second transmission medium having different characteristic impedances. For instance, this is the case where a connector electrically couples a PCI bus to a peripheral card. Another source of reflection is where a single transmission path splits into two or more transmission paths.
Thus, the addition of connectors and riser cards between onboard devices and PCI devices on peripheral cards causes a major signal quality problem on the 66 MHz PCI bus. As transmission frequencies increase, the disruption of signal waveforms caused by such reflection becomes increasingly problematic. Hence, in designing a computer board that includes a 66 MHz bus, transmission paths, including the sockets, riser cards, and peripheral cards, must be carefully considered in order to minimize signal reflection.
Accordingly, there is a need for a compact riser card system which will provide two or more 64-bit PCI sockets to couple one or more peripheral boards to a 66 MHz PCI bus on a computer board while satisfying the height requirements for 1U systems.